Influenza is a disease caused by influenza viruses infecting the respiratory organs. In general, influenza causes a fever of 38° C. or higher in an individual infected with an influenza virus after a latency period of about 1 to 2 days, which is accompanied by general symptoms such as a headache, general malaise, and joint and muscle pains. Respiratory symptoms such as coughing and sputum follow, but the condition usually improves within a week. However, when influenza infects individuals such as the elderly, infants, expectant mothers, patients with chronic respiratory or circulatory system diseases, diabetics, or patients suffering from chronic kidney failure, serious and sometimes fatal complications such as pneumonia and bronchitis may develop. Moreover, influenza, which causes serious health problems, is also highly contagious, and numerous people become infected in a short period of time, resulting in enormous social and economical losses.
The administration of influenza vaccines is the most effective way to prevent health damage caused by influenza infections, and to reduce social and economical losses. Influenza vaccine preparations such as liquid preparations for use as injections and frozen preparations for use as nasal drops have been known in the past; however, no dry preparations of satisfactory stability have yet been made available to the market (see, for example, Non-Patent Document 1).
When influenza vaccines are distributed in the form of liquid preparations, they must always be maintained at low temperatures during distribution and preservation, in order to prevent the vaccines from becoming inactive; that is, cold chains are indispensable. Additionally, when influenza vaccines are distributed in the form of frozen preparations, it is necessary to maintain the vaccines in a frozen state during distribution and preservation in order to stably keep the preparations. The influenza vaccines that have been made into liquid or frozen preparations thus require advanced temperature control during transportation and preservation (see Non-Patent Document 2), making it difficult to distribute these preparations in areas with power supply shortages or without low-temperature transportation while maintaining the activity of the influenza vaccines.
In order to overcome such drawbacks of liquid or frozen preparations, it is effective to distribute influenza vaccines in the form of dry preparations. Although a technique concerning freeze-dried preparations of influenza vaccines has recently been proposed wherein influenza vaccines are made into preparations with the addition of lactose or trehalose, the stability of such preparations is nowhere indicated (see Non-Patent Document 3).
Like other vaccines, influenza vaccines are highly susceptible to heat, and are known to lose activity at high temperatures or below their freezing point (see Non-Patent Document 2). Even if the influenza vaccines are simply dried or freeze-dried, their activity problematically decreases with time during production or preservation. A technique to overcome this problem has yet to be found, and no successful examples of putting dry preparations of influenza vaccines into practical use have been reported to date.    Non-Patent Document 1: Deborah A. Buonagurio et al., Vaccine, vol. 24 (2006), 2151-2160    Non-Patent Document 2: Stephen E. Zweig, Vaccine, vol. 24 (2006), 5977-5985    Non-Patent Document 3: Robert J. Garmise et al., AAPS PharmSciTech 2006; 7 (1) Article 19, E1-E7